This invention relates to a measuring apparatus of a volume of fluid such as fuel injected from an injection valve. More specifically, this invention relates to an apparatus which precisely measures the volume of the fluid injected from an injection valve to be measured under the conditions conforming to the actual injection of the fluid.
When the injection valve supplies fuel to a combustion chamber of an engine, for example, a diesel engine and an electronically-injected gasoline engine, the injected fuel volume should be precisely controlled. Consequently, for the design, development and inspection of such a control system, it has previously been proposed to provide a precise measuring apparatus of a volume of an injected fuel.
When the injection valve injects fuel into a combustion chamber in which pressure is maintained at a predetermined value, the injected volume of the fuel is determined by measuring the increment of the chamber volume (Japanese published examined patent application No. 58-37485). The injection chamber is pressurized in advance to avoid a measurement error caused by the contraction of gas mixed in the fuel injected into the chamber.
In this method, the increment of the injection chamber volume is determined by measuring the displacement of the piston. As an additionally disclosed method, the injection chamber is partitioned with bellows (or a diaphragm) and a circular plate, and the displacement of circular plate determines the volume of the fuel injected into the injection chamber. This method has advantages; fluid does not leak from the injection chamber, the inertia force of a movable member used in this method is small, and no friction arises between the circular plate and the wall of the injection chamber.
In the diesel or gasoline engine cylinder into which the injection valve to be measured actually injects fuel, the pressure therein becomes high, varying in a wide range between 30 and 150 kg/cm.sup.2. The pressure in the injection chamber influences the volume of injected fuel. Consequently, when the pressure in the injection chamber is maintained constant as aforementioned, the problem is that the volume of fuel injected under actual injection condition cannot be precisely measured.
The bellows or diaphragm system has advantages as described above. But when the bellows or diaphragm deforms slightly as the pressure in injection chamber rises at the time of fuel injection, the problem is that the increment of the injection chamber volume is not proportional to the displacement of the circular plate. Further, the injection pressure transversely displaces or inclines the bellows, resulting in a measurement error.
In the prior art, to measure the displacement of the circular plate, the movement of a rod attached thereto is detected by a differential transformer. But as the rod displaces with the circular plate, the inertia force of the movable portion is large, resulting in an ineffectiveness for measuring the volume of the fuel injected from the injection valve at high frequency. Moreover, temperature drift increases because of the thermal expansion of the rod. For a zero point adjustment, the circular plate contacts a stopper while the fluid is not injected. Measurement of the volume of the fuel injected from the injection valve at high frequency is difficult due to the vibration caused by such a contact.